NASA officials said Tuesday they intend to award multiple companies up to $500 million each over the next two years to maintain private sector competition in the agency's commercial crew program and keep the effort on schedule to enable the resumption of U.S. human spaceflight by 2017.

The agency released an announcement for proposals to industry Tuesday, starting the third phase of the commercial crew program, which aims to invest in the development of spacecraft and launch vehicles capable of hauling astronaut crews to the International Space Station.

The commercial crew program is the key to ending NASA's reliance on Russian Soyuz spacecraft for human transportation to and from the space station since the retirement of the space shuttle. NASA officials expect commercial crew service to begin no later than 2017, based on current budget projections.

Ed Mango, manager of NASA's commercial crew program, said the space agency plans to award Space Act Agreements, or SAAs, to multiple contractors by August. The agreements will each be worth between $300 million and $500 million, and NASA will make payments as the companies accomplish predetermined milestones.

Brent Jett, NASA's deputy commercial crew program manager, said he is confident the agency will be able to award at least two companies agreements of that value.

Proposals are due March 23, according to the solicitation.

"What our overall approach is during this SAA is we are an investor, a technical investor and an investor bringing money to the table in order to get American industry to get to a crew demonstration capability," Mango said Tuesday at a forum with members of industry at the Kennedy Space Center in Florida.

The Commercial Crew integrated Capability, or CCiCap, phase of the program will mature the designs of combined space transportation systems, which include a spacecraft to carry at least four astronauts and a launch vehicle to safely dispatch the crews from Earth.

The objectives of the CCiCap agreements are to complete the design of the commercial spaceships and ready the vehicles for construction and certification.

NASA planned to issue a commercial crew solicitation in December for fixed-price contracts, but the agency's fiscal year 2012 budget provided for $406 million for the commercial crew program, less than half the funding level requested by the White House.

The smaller budget, coupled with uncertain funding in future years, compelled NASA officials to revert to Space Act Agreements, which have been used in the first two phases of the commercial crew program.

NASA now has funded Space Act Agreements with Boeing Co., SpaceX, Sierra Nevada Corp. and Blue Origin worth a combined $316 million. Each company is working on a commercial crew spacecraft, and the agreements run through May.

"I can tell you that since November, the program has had to pivot because of a number of different things," Mango said. "The biggest is uncertainty in the future of our budget."

The Space Act Agreements will run at least 21 months from August 2012 until May 2014, roughly the same period the contracts would have covered.

"The overall net plan that we have in place will still mitigate the risks needed to understand certification in order to go fly crew safely," Mango said.

In the coming competition, the commercial partners will also be expected to demonstrate processes to analyze, quantify and understand safety risks, institute procedures and testing to reduce the risks, and propose criteria and plans to certify their spacecraft and rockets for human spaceflight.

NASA is requesting companies provide lists of optional milestones extending beyond May 2014, both at a fixed funding profile of $400 million per year and an optimum, more costly, profile with an accelerated schedule to begin commercial crew service by the middle of the decade.

"We think the most we might be able to give [the commercial partners] is about $400 million a year [each]," Mango said.

The optional milestones must outline continued development through a crewed orbital test flight, the final step before a transportation system is declared ready for round-trip service to the space station.

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